Maria Sharapova rued a colossal 52 unforced errors as she slumped to a first-round defeat at the Qatar Open on Monday.

The former world number one and two-time champion in Doha was making her first appearance at the event in five years, but her return ended in disappointment as Monica Niculescu battled back from a set down to win 4-6 6-4 6-3.

Sharapova had looked settled in the opening set and was producing her powerful trademark groundstrokes from the baseline, and when she went 1-0 ahead things looked promising.

However, errors began to creep into her game and Niculescu took full advantage to record an opening-day upset, leaving Sharapova to provide an honest assessment of her performance.

"I thought I did a really good job in the first set and a half, but that's obviously not enough to win the match," Sharapova said.

"I did a good job of winning the longer rallies, even though that's not really what I wanted to get myself into.

"So physically I felt good. I just got pretty passive in the end and started making too many errors.

"I think I have to look at the tape and see why I started going back behind the baseline and not moving forward, not putting the pace on her."

Niculescu could meet top seed Caroline Wozniacki in round three, but will have to get past either Magdalena Rybarikova or Fatma Al Nabhani first.

Five-time semi-finalist Agnieszka Radwanska wasted nine match points before eventually seeing off Mona Barthel 3-6 6-3 7-5.

Seventh seed Caroline Garcia will face Dominika Cibulkova in round two after she battled past Anastasia Pavlyuchenkova – the Slovakian taking the first set tie-break 10-8 before wrapping up the match 6-4 in the second.

Monday also saw straight-set victories for Duan Ying-ying, Miheala Buzarnescu, Ekaterina Makarova and qualifier Anna Blinkova, while Naomi Osaka battled through back pain to beat Katerina Siniakova 6-4 6-0.

Cici Bellis is also through after Daria Kasatkina was forced to retire at 7-5 4-1 down, the Russian struggling with a neck problem after a fall in the first set.